142 PHYTOHORMONES 



decapitation. It is thus clear that if the experiments are car- 

 ried out carefully and not too much of the tip is removed, 

 all of the roots studied — with the exception of those of 

 Pisum — show a definite acceleration after decapitation (c/. 

 also Cholodny, 1933). 



In an attempt to explain this phenomenon, Cholodny 

 (1926, 1931a) replaced the tip on maize roots and found that 

 their growth was again retarded. Coleoptile tips had pre- 

 viously been found to behave in the same way (1924). 

 Keeble, Nelson, and Snow (1931a) confirmed these results. 

 Further, when the tip is removed, the root stump loses its 

 sensitivity to gravity, as has been known since Darwin 

 (1880), but if root or coleoptile tips are replaced on the 

 stump Cholodny (1924) found that its geotropic sensitivity 

 is largely regained. This suggests, as Cholodny (1926) 

 pointed out, that both the root tip and the coleoptile tip 

 secrete a substance retarding root growth, and that this 

 substance enables the root to give a tropistic response by 

 becoming more concentrated on one side than the other 

 (c/. X5). From its production by coleoptile tips it might 

 be supposed that this growth-retarding substance is identical 

 with auxin. 



B. The Effect of Auxin on Root Growth 



The above possibility was strongly borne out by the ex- 

 periments of Nielsen (1930), Navez (1933), and Boysen 

 Jensen (19336) (see also Keeble, Nelson, and Snow, 1930). 

 They immersed roots of various plants in water and in 

 difTerent concentrations of Rhizopus culture medium (which 

 contains indole-3-acetic acid, cf. VII C). The growth of the 

 roots was very greatly retarded by the active culture me- 

 dium. That this retardation is really due to the auxin and 

 not to the other substances present was shown by Kogl, 

 Haagen Smit, and Erxleben (1934a) by immersing Arena 

 roots in solutions of auxin a, auxin b, and indole-3-acetic 

 acid, when they were strongly retarded. The extent of in- 

 hibition is proportional to the concentration of auxin, and 



